How to use the Samsung S5 emergency power.?

No such thing.

Another urban legend.

I am asking the question about Samsung S5 but I think it also apply to the other models of Samsung phones. When my phone battery was exhausted, I plugged in the USB charge cable with the phone turned off, and I would see the remaining battery level at 33%. You would expect 0% or like 5%. Someone told me that the additional power is hidden from the user for some other purposes, like for someone to spy on you. So like if the FBI want to monitor your conversation, even if you phone is powered off, they can still tap you. Not sure if that is true, but I heard that you can still wake up the reserve power for emergency. If you know how to do this please share with me.. Thanks.